Title: The relationship between match statistics and top 100 ranking in professional men's tennis
Machar Reid, Darren McMurtrie, Miguel Crespo
International Journal of Performance Analysis in Sport 07/2010; 10(2):131-138.

Abstract
Match statistics are supplied at the majority of professional tennis tour events. The governing body of the men's tour - the Association of Tennis Professionals (ATP) - updates players' statistical profiles on a weekly basis. The relationships between the rankings and fourteen statistics describing the match performance of the top 100 male professional players in 2007 were examined to determine which statistics were most related to playing success. Partial correlations determined the strength of these associations and selected variables were entered into a stepwise regression procedure to predict professional ranking. Five variables were significant predictors of top 100 ranking while only second serve return points won and second serve points won remained in the final prediction equation, which accounted for 52% of the variance in professional ranking: predicted men's professional ranking = 548.5 + -666.6 * second serve points won + -319.9 * second serve return points won. This analysis suggests that second serve points won and second serve return points are among the most relevant statistics commonly available to ATP players.

Title: The relationship between match statistics and top 100 ranking in professional men's tennis
Machar Reid, Darren McMurtrie, Miguel Crespo
International Journal of Performance Analysis in Sport 07/2010; 10(2):131-138.

Abstract
Match statistics are supplied at the majority of professional tennis tour events. The governing body of the men's tour - the Association of Tennis Professionals (ATP) - updates players' statistical profiles on a weekly basis. The relationships between the rankings and fourteen statistics describing the match performance of the top 100 male professional players in 2007 were examined to determine which statistics were most related to playing success. Partial correlations determined the strength of these associations and selected variables were entered into a stepwise regression procedure to predict professional ranking. Five variables were significant predictors of top 100 ranking while only second serve return points won and second serve points won remained in the final prediction equation, which accounted for 52% of the variance in professional ranking: predicted men's professional ranking = 548.5 + -666.6 * second serve points won + -319.9 * second serve return points won. This analysis suggests that second serve points won and second serve return points are among the most relevant statistics commonly available to ATP players.

I'd say these are the stats that usually separate the top guys from the rest. But of course they need to filter out guys with too much clay court matches since that skews things. I wish their was something similar to sabermetrics in tennis.

So why do players practice their serves all the time, but only rarely "volunteer" to practice their returns against their hitting partners - and gain the experience in returning so necessary to their game?
[Two players can pick up balls during serve/return practice more quickly than one player can.]